1. Field of the Invention
This invention relates to passive acousto-optic modulators for use, in particular, in microphones for divers.
2. Discussion of the Prior Art
Optical fibers are generally used for very large bandwidth, high data rate digital communication systems. However, optic fibers can also be used as analogue low bandwidth verbal communication systems for divers. One or two optic fibers are fed from the surface down to the diver. Light generated from a constant illumination source is passed down the fiber. The diver speaks into a microphone which modulates the light signal. The modulated light signal is then returned to the surface. The returned signal is then converted into an electrical signal, the signal then being amplified and demodulated, the resulting signal being used to generate the verbal message.
Existing designs of optical microphone comprise acousto-optic modulators which modulate an optical signal in accordance with an acoustic signal. To avoid any additional cabling, it is desirable that the microphones are able to operate without the need for a power supply.
Presently, acousto-optic modulators comprise a series of lenses and a reflective diaphragm. When two optical fibers are used, the acousto-optic modulator comprises an additional beam splitter. The light from the constant illumination source passes down an optical fiber and into the microphone. A convex lens on the end of the fiber focuses the light into a parallel beam. The light is then focused onto a reflective diaphragm by a second convex lens which then reflects the light back along the path it has just travelled. If a beam splitter is used, the incoming beam of light from the source is separated from the reflected light so that each can be sent via a different optical fiber. The position of the diaphragm at rest is fixed relative to the second lens. If the diaphragm is situated at the focal point of the lens, then the amount of light reflected back is equal to the intensity of the light at the focal point. As the diaphragm is moved from the focal point, the intensity of light reflected back is attenuated. Therefore, the light becomes modulated as a function of the movement of the diaphragm. The diaphragm is moved by the varying air pressures generated by the diver's speech.
However, there are several problems with this design of microphone. Firstly, the microphone is an "in air" microphone, capable of only operating in a dry environment. The design is vulnerable to water ingress, particularly when the diver is using it at depth where high pressures would be applied to the microphone. Droplets of water on the lenses cause distortions in the light signal and therefore lose of sensitivity. Total flooding of the microphone will cause the focal lengths of the lenses to change making the microphone very insensitive or not operable at all.
Secondly, each of the air/glass interfaces of the lenses act as an attenuator to the light signal, loosing around 3dBs per interface. This leads to insensitivity of the microphone and noisy return signals.
U.S. Pat. Nos. 4,926,696, 4,942,767 and 5,052,228 disclose three different devices which modulate a light signal but operate on the same principle to each other. The devices comprise a diaphragm which is located so that it faces a beam splitter. A light signal passes through the beam splitter and is separated into two light signals. One or the light signals proceeds to the diaphragm where it is reflected back. The reflected light signal then constructively or destructively interferes with the other signal to create a modulated light signal. The amount of constructive or destructive interference is dependent on the distance between the diaphragm and the beam splitter. Therefore, the amount of modulation of the light signal is dependent on the movement of the diaphragm. The diaphragm movement is dependent on pressure applied to the reverse side of the diaphragm. In U.S. Pat. Nos. 4,942,767 and 5,052,228 the beam splitter is formed by the end of an optic fiber.
U.S. Pat. No. 5,247,490 discloses an acoustic optical sensor which operates on the same principles as the devices disclosed in U.S. Pat. Nos. 4,926,696, 4,942,767 and 5,052,228. However the device has been pressure compensated for operation in high pressure environments such as the deep sea. The document Deport Telephonique Sur Fiber Optique by Ecole Centrals De Paris discloses a microphone which converts sound vibrations into an modulated optical signal using the same technique.
UK Patent Application GB 2,117,993 discloses an acoustic optical modulator which comprises a piezo electric hydrophone to convent acoustic vibrations into an electrical signal, the electrical signal then being used to drive a liquid crystal modulator which modulates a light signal as a function of the acoustic vibrations.
U.S. Pat. No. 4,514,860 discloses a sensor system for measuring primarily electric voltages or magnetic fields and comprises an optical fiber which is arranged so that the end faces a luminescent layer, a number of optical filters and two electrodes which generate within the luminescent layer a lateral electric field for the purpose accelerating charge carriers in the layer which are excited by light shone onto the luminescent layer via the fiber optic.